In high temperature electrolyzers, electrolysis of water at a high temperature is achieved from vaporized water. The function of a high temperature electrolyzer is to transform the steam into hydrogen and oxygen according to the following reaction: 2H2O(g)→2H2+O2.
This reaction is carried out via an electrochemical route in the cells of the electrolyzer.
Each elementary cell is, as this is seen in FIG. 1, made up with two electrodes, i.e. an anode (1) and a cathode (2), sandwiching a solid electrolyte generally in the form of a membrane (3).
Both electrodes (1, 2) are electron conductors and the electrolyte (3) is an ion conductor.
The electrochemical reactions occur at the interface between each of the electron conductors and the ion conductor.
At the cathode (2), the half reaction is the following: 2H2O+4 e−→2H2+2O2−;
And at the anode (1), the half reaction is the following: 2O2−→O2+4e−.
The electrolyte (3) placed between both electrodes is the migration site of the O2− ions (4), under the effect of the electric field generated by the potential difference imposed between the anode (1) and the cathode (2).
An elementary reactor illustrated in FIG. 2, consists of an elementary cell (5) as described above, with an anode (1), an electrolyte (3), and a cathode (2) and two monopolar connectors or more exactly two half-interconnectors (6, 7) which ensure electric, hydraulic and thermal functions. This elementary reactor is called a module.
In order to increase the produced hydrogen and oxygen flow rates, as this is shown in FIG. 3, several elementary modules are stacked (8), the cells (5) then being separated by interconnectors or bipolar interconnection plates (9).
The assembly of the modules (8) is positioned between two upper (10) and lower (11) interconnection plates which bear electric power supplies and gas supplies (12). One then refers to this as a stack (FIG. 3).
There exist two concepts, configurations, architectures for the “stack”:                tubular stacks, in which the cells are tubes, and        planar stacks, in which the cells are made as plates like in FIG. 3.        
In the planar architecture, the cells and the interconnectors are in contact in many points. The manufacturing of the stack is subject to fine tolerances as to the flatness of the cells in order to avoid too high contact pressures or an inhomogeneous distribution of the stresses, which may lead to cracking of the cells.
The sealing joints (gaskets) in a stack have the goals of preventing a hydrogen leak from the cathode towards the neighboring anodes, preventing an oxygen leak from the anode towards the neighboring cathodes, preventing a hydrogen leak towards the outside of the stack and finally limiting the leaks of steam from the cathodes to the anodes.
Within the scope of the development of a stack for high temperature electrolysis (“HTE”), and as is shown in FIG. 4, gas-tight joints (gaskets) (13) are thus made between the planar electrolysis cells (5), each consisting of an anode/electrolyte/cathode ceramic trilayer, and the interconnectors or metal interconnection plates (9).
It should be noted that the dimensions given in μm in FIG. 4 are only given as examples.
More specifically, a joint (gasket) is made between the lower surface of each cell (5) and the upper half-interconnector (14) of the interconnection plate located below the cell on the one hand, and between the upper surface of each cell and the lower half-interconnector (15) of the interconnection plate located above the cell (5) on the other hand.
These joints (gaskets) (13) should generally have a leak flow rate in air below 10−3 NmL/min/mm between 700° C. and 900° C. under a pressure difference, gap, from 20 to 500 mbars.
In addition to this sealing function, the joint (gasket) may in certain cases have secondary assembling and electric conduction functions. For certain stack architectures, a ceramic part, called a cell support, may be placed between the cells and the interconnectors; and gas-tight joints (gaskets) are then also required with this cell supporting part.
Several sealing solutions are presently being studied, i.e.: cements or ceramic adhesives, glass joints (gaskets) or glass ceramic joints (gaskets), compressed metal joints (gaskets), compressed mica joints (gaskets), brazed joints (gaskets) and mixed solutions requiring several of these techniques.
These joints (gaskets) should make it possible to ensure the seals between the cathodic chamber and the outside, between the anodic chamber and the outside, and between both chambers, and thereby avoid gas leaks between both chambers and towards the outside.
The seals by brazing are generally achieved between dense materials which are the electrolyte (3), for example in yttriated zirconia on the one hand and the interconnectors (9, 14, 15) or the cell supports on the other hand.
In the case of high temperature fuel cells (SOFC) in which the support is formed by the electrolyte and which are thereby called “electrolyte supported cells” (“ESC”), the electrodes are of smaller dimensions than the electrolyte so that the brazed gaskets made at the periphery are not in contact with the electrodes.
Also, industrially, for Anode-Supported Cells (“ASC”), the dimensions of the cathode are themselves reduced in order to be able to braze the interconnector onto the electrolyte, since the interconnector and the electrolyte are both formed by dense materials. Therefore there then exists a certain drawback related to the surface area loss of the electrodes.
Indeed, the electrodes, the anode and the cathode, are porous materials, having generally a porosity of the order of 30-50% by volume and brazing of such porous materials has many difficulties and many drawbacks.
Although certain patent applications, for example applications WO-A1-2006/086037; WO-A2-2006/127045; WO-A2-2007/062117 mention the possibility of brazing porous electrodes, no demonstration of the feasibility of such a method without degrading the electrodes is made at the scale of the grain.
More specifically, if an attempt is made to achieve a brazed gasket between these porous electrodes and the interconnectors in order to ensure a tightness in the direction of the thickness of the electrodes, the brazing alloy infiltrates by a capillary effect the pores over very large distances, which may laterally attain, reach, for example several mm, which reduces their electrochemically active surface area and thus reduces their yield.
By reducing the brazing temperature in order to make the alloy viscous, it is possible to manage control of this infiltration in the electrodes.
But, for a stack, this requires perfect homogeneity of the temperature over the whole of the stack, which industrially is very difficult to control.
Today, the thickness of the porous electrodes is defined to within ±10 μm by the suppliers and the rated nominal dimension may change over time or be modified.
In front of these risks, and despite the problems mentioned above, the choice was therefore made to make the interconnector/cell seals on the porous electrodes. With this orientation, it is possible to notably simplify the geometrical specification of the cells.
However, in order to properly control the chain of dimensions of a electrolysis stack and thus maintain all the electric contacts between the interconnectors and the electrodes, a limited overthickness or even no overthickness has to be generated by the brazed gaskets at the interconnector/cell interface.
If an overthickness is inevitable, it requires either perfect control of a constant thickness of the brazed joints for each cell, or the addition of thickness shims, also called spacers, or further machining or Stamping of the interconnectors with extreme accuracies on their geometrical tolerances.
The first solution is not at all under control, while the second and third solutions complicate the manufacturing method and should be avoided.
Considering the foregoing, there therefore exists a need for a method for manufacturing a high temperature electrolyzer or a high temperature fuel cell comprising a vertical stack of elementary planar cells separated by interconnection plates, gas-tight brazed gaskets achieving the assembling of the cells and of the interconnection plates, in which said gaskets are made between the interconnectors and the porous electrodes, and in which the infiltration of the brazing composition into the porous electrodes is perfectly under control, in all the directions, and notably laterally, so as to ensure a mechanically solid assembly of the whole “stack” and not to reduce the electrochemically active surface area of the electrodes.
There still exists a need for such a method which allows perfect control in a simple and reliable way, of the chain of dimensions (tolerance stack-up) of the stack, such as its total thickness, in order to thereby maintain all the electric contacts between the interconnection plates and the electrodes.
In particular, there exists a need for a method with which stacks may be made in which the brazed gaskets have no overthickness, in other words in which the upper or lower portion of these gaskets remains in the plane of the electrodes which is also the one of the interconnection plate (or of the ceramic support) to be assembled.
There also exists a need for such a method which is simple, reliable, only includes a limited number of steps and which avoids resorting to complex steps, difficult to control or costly.
The goal of the present invention is to provide a method for manufacturing a high temperature electrolyzer comprising a vertical stack of n elementary planar cells alternating with n+1 interconnection plates, each of the elementary cells being composed of (consisting in) a planar porous anode and a planar porous cathode respectively positioned on each of the faces of a planar dense electrolyte, and gaskets being provided at contact points between the elementary cells and the interconnection plates, which meets the needs listed above.
The goal of the present invention further is to provide such a method which does not have the drawbacks, limitations, defects and disadvantages of the compositions of the prior art and which solve the problems of the methods of the prior art.